This invention relates to r.f. mixers, i.e. for frequency-changing an input signal.
The invention is particularly concerned with mixers using FET switching devices for connecting the input and output, e.g. FETs arranged in a double balanced, i.e. bridge form.
The alternate reversing of the connections between the input and the output may be effected by driving the gate electrodes of the FETs in each pair of opposite branches in anti-phase at a local oscillator frequency.
In order to obtain high linearity in the mixer performance, the FETs must be switched as rapidly as possible, and this implies using a square wave to drive the gate electrodes.
A transformer has been used to provide such a drive, but a square wave requires a broad bandwidth if the rise and fall times are to be rapid. This can only be achieved if the transformer is terminated in a low impedance but, since the output impedance of the gates is capacitive, this requires a defined low resistance shunted across the output of the transformer drive, which would dissipate power.
One way in which a low power drive input to the FETs has been achieved is by employing a resonant circuit of which the capacitances of the FETs form a part (Electronic Engineering, May 1986, Ed Oxner, "High Dynamic Range mixing with the Si 8901", pages 53-56), but then a square wave drive could not be used.